1. Field of the Invention
The invention relates to electrical impedance matching systems, and particularly to systems which use electrical power to generate a plasma.
2. Discussion of the Background
In many electrical device and solid state manufacturing processes, a plasma is utilized to react, or to facilitate a reaction, with a substrate such as a semiconductor wafer. To generate a plasma, RF power can be supplied to a gas by a capacitive and/or an inductive plasma coupling element. For example, an electrode can be provided as capacitive coupling element, while a conductive loop or coil can be provided as an inductive (i.e., magnetic) coupling element.
If the impedances of the power source and the load (i.e., the element coupled to the plasma) are not matched, the power supplied to (or absorbed by) the load is not maximized, and it can -be difficult to control the amount of power absorbed by the load. In addition, unmatched impedances can be detrimental to the power source or to the components which are coupled to the power source. In most cases the load impedance (i.e., the input impedance of the plasma coupling element) cannot be determined in advance, since it is dependent on the state or condition of the plasma to which it is coupled, and the plasma state can vary during processing. Accordingly, many plasma processing systems utilize a matching network provided between the RF source and the plasma coupling element to match the impedances. The matching network is utilized in order to maximize the amount of RF power supplied to the plasma, and to control the amplitude and phase of this power.
Over the past thirty plus years, automatic impedance matching networks have been designed and implemented to maximize the transfer of power between a source power supply and load. Those networks are commonly found in the application of RF power to plasma generation in semiconductor processing (see Beaudry, U.S. Pat. No. 3,569,777; Seward, U.S. Pat. No. 4,112,395; Meacham & Haruff U.S. Pat. No. 4,557,819; Collins et al., U.S. Pat. No. 5,187,454; Shel, U.S. Pat. No. 5,585,766; Smith et al., U.S. Pat. No. 5,621,331; Richardson et al., U.S. Pat. No. 5,689,215; Barnes & Holland, World Patent WO9724748). They are also found in the application of VHF/UHF power to plasma generation in semiconductor processing (see Collins & Roderick, U.S. Pat. No. 5,065,118), and the application of microwaves to plasma generation in semiconductor processing (see Kingma & Vaneldik, U.S. Pat. No. 3,617,953; Rogers, U.S. Pat. Nos. 3,745,488, 5,041,803; Ishida & Taniguchi, U.S. Pat. No. 5,079,507; U.S. Pat. No. 5,543,689), and RF antenna broadcast (see Kuecken, U.S. Pat. 3,601,717; Templin, U.S. Pat. No. 3,794,941; Smolka, U.S. Pat. Nos. 3,825,825; 3,919,644; Straw, U.S. Pat. No. 3,959,746; Brunner, U.S. Pat. No. 3,995,237; Ott, U.S. Pat. No. 4,004,102; Armitage, U.S. Pat. No. 4,356,458,; Theall, U.S. Pat. No. 4,375,051; Collins, U.S. Pat. No. 4,951,009; Gubisch, U.S. Pat. No. 5,057,783; Roberts & DeWitt, U.S. Pat. No. 5,225,847; Flaxi, U.S. Pat. No. 5,491,715).
One of the first patents to address the automation of impedance matching in plasma processing is Beaudry (U.S. Pat. No. 3,569,777). It uses mechanically adjusting variable capacitors and inductors. Stimson (U.S. Pat. No.5,629,653, Applied Materials, Inc.) and Mazza (Automatic Impedance Matching system for RF sputtering, IBM J. Res. Devel. p 192, 1970) also use two capacitors to tune the impedance mismatch.
In general, the measurement of power transmitted to the load has been completed from a measurement of the voltage, current and phase. Conventionally, the feedback control algorithm iteratively adjusts the capacitances until either the transferred power is maximized or the reflected power is minimized and has been applied to matching networks employed in the coupling of RF power to plasma devices for semiconductor processing. Methods of detecting the impedance mismatch between the source and the load are (a) to monitor the voltage standing wave ratio (see Templin, U.S. Pat. No. 3,794,941; and Brounley, U.S. Pat. No. 5,473,291), and (b) to monitor the impedance of the plasma reactor (see Bouyer et al., U.S. Pat. No. 4,990,859),
An example of a known matching network is illustrated in FIG. 9, in which a matching network MN matches the impedance of a power source 2 with that of a load 3. The matching network, which is connected between a cable 70 and the load 3, includes a constant inductor L and variable capacitors C1 and C2. The matching network is tuned using servomotors 80A and 80B to vary the capacitance of the variable capacitors. Alternatively, the impedance of a variable inductor can also be controlled. The servomotors 80A, 80B are driven by a matching controller (not shown), which also monitors the quality of matching using electrical connections within the matching network. By accurately controlling the plasma-generating power, the plasma conditions are more controllable and reproducible, thereby improving the yield and accuracy of the process, and preventing damage to the power source or other components of the system.
Several patents have been issued that address some aspects of intelligent control of impedance matching. In fact, most patents proposing automatic impedance matching require some form of an "intelligent" controller (such as those referenced above). In general, the intelligent systems attempt to obtain some correlation between settings for variable reactances (i.e., capacitors and inductors) and plasma conditions such as the load impedance or plasma chamber input parameters (i.e., RF input power, chamber pressure, etc.). Through obtaining such a correlation, fast, stable coarse tuning of the impedance matching can be obtained. For example, Keane & Hauer (U.S. Pat. No. 5,195,045) present a method of using predetermined set points for two impedance varying devices in order to solve tuning problems during run conditions. Additionally, Ohta & Sekizawa (U.S. Patent No. 5,543,689) have proposed storing match circuit settings from prior use; in some sense, characterizing their system a priori. Others attempt to use an estimated load impedance and a measured input impedance to the network wherein coupling the information forms a predictive-corrective algorithm for a tuning control function (Collins et al., U.S. Pat. No. 5,187,454). Smith et al.(U.S. Pat. No. 5,621,331) have presented a notable method for rapidly adjusting the impedance of a variable impedance device to match the impedance of a source to the impedance of a load in a plasma processing device. The device includes a plurality of electrical sensors, photo-sensitive detector, a data processor and a memory. In particular, measurements of chemical species present within the plasma using an optical emission spectrometer and electrical measurements taken on plasma coupling elements have been correlated with variable reactance settings.
Neural networks have been used for both predication and control in many areas. A use of neural networks in semiconductor processing to predict the endpoint of an etch process is discussed by Maynard et al. in "Plasma etching endpointing by monitoring RF power systems with an artificial neural network," Electrochem. Soc. Proc., 95-4, p 189-207, 1995, and "Plasma etching endpointing by monitoring radio-frequency power systems with an artificial neural network," J. Electrochem. Soc., 143 (6).
Conventional matching systems have a number of shortcomings. For example, as solid state device processing technology has advanced, the plasma processing systems have become more complex, requiring multiple plasma coupling elements. In conventional systems, the coupling elements have been supplied with RF power through separate matching networks controlled by independent matching controllers. However, with independent matching controllers, control of the matching conditions can become unstable, as one controller attempts to tune one matching network, while another controller attempts to tune another matching network. If multiple plasma coupling elements are all coupled to the same plasma, the controllers can interfere or compete with each other and, in severe cases, can cause uncontrolled oscillation of the servomotors controlling the variable capacitors. Even if uncontrolled oscillations do not result, the multiple matching controllers can be slow in reaching a matched condition, since tuning of one matching network will affect the matching of a coupling element associated with another matching network, requiring further matching/tuning until all of the matching networks have converged upon a matched condition. In fact, difficulties in simultaneously matching multiple plasma coupling elements have caused some manufacturers to avoid using variable matching networks altogether. Instead, these manufacturers use either no matching networks or non-tunable matching networks, and the frequencies of the RF sources are tuned in order to optimize the coupling to the plasma. One of the disadvantages with this approach is that it can be very expensive to purchase multiple, independently frequency-tunable RF sources. Therefore, there is a need for improved matching networks, and particularly for improved control of systems having plural matching networks
Another problem with conventional systems is that they may require highly skilled operator personnel, especially when several different processes are used, each with a different set of operating conditions. Of particular importance are the skills required to initiate a plasma and to tune the matching networks to their initial conditions immediately after plasma initiation. In conventional systems, these steps require an experienced, highly trained operator (the operator makes manual adjustments until the system is approximately matched, and then the operator turns on the matching controller). However, skilled operators are often not readily available and, in order to reduce labor costs, manufacturers have a desire to employ less skilled operators. As a result, the matching controllers should be more intelligent, require less operator supervision, and be less susceptible to operator error.
There are also several important limitations of the instruments currently used to measure the matching of the RF source to the plasma coupling element(s). For example, conventional phase detectors have a poor dynamic range, due to low sensitivity at low voltage levels and due to overloads of the circuitry at high voltage levels. In addition, conventional peak detectors, used to determine the amplitude of voltages at the matching networks, have a poor dynamic range, due to voltage offset errors caused by diodes used in their circuitry. These offset errors can reduce the sensitivity of the peak detectors at low voltage levels.
Conventional systems also rely on forward/reflected power detection at the power source itself, and therefore can suffer from errors caused by poorly characterized, or even damaged, cables. Accordingly, another deficiency of conventional systems is the reliance upon forward/reflected power detection at the power source.
Several problems are also associated with the circuits currently used to control the motors that tune the capacitive elements in the matching networks. In addition to being overly complex and expensive, conventional control circuits offer insufficient protection against damage to the capacitors, which can be caused by mechanical components being forced to travel beyond their acceptable range of operation. Moreover, conventional servomotor arrangements and associated circuitry can be slow in starting and/or stopping movement when tuning the variable impedance elements.
Accordingly, improved plasma power control systems are needed which provide matched power and load impedances and which avoid the foregoing shortcomings. In addition, an improved control is needed for systems which include multiple matching networks.